Oscar not growing and a reduced appetite

Question
My Oscar Leuchen that is about 8-10 months old has not grown in 4 months, he/she remains about 3.25 inches. The tank is 170L, 24 degrees C and running a 2-300 L/h filter, up until a week and a half ago was an absolute guts, plenty of energy, happy, still no growth... he/she has a best friend, another Oscar Sookie, they get along like a house on fire, no fights or displays of aggression at all, I've had Sookie for 5 months and has doubled in size as expected. I've watched them feed and am very sure they get even amounts..... i am worried there is something more wrong with leuchen than a parasite, he/she seems so sad and mouths the word 'hello' all the time at me (something that both the fish picked up after a while, this became a feeding call for them as i called hello to them at feeding time!!)  leuchen has not stopped completely eating but he is extremely fussy with what's given, barely eating anything, Sookie's behavour has  remained the same!  i have treated all the fish for parasite with tablets give from my local Aquarium and specific instructions on how to administer them  including a partial water change etc. Checked and maintained Ph levels at 6.6-6.8, ammonia, nitrates/ites at safe level and still no change in behavior :(   What can/should i do?

Answer
Hi Rhian,

My guess is parasites.  Use PraziPro to dose the tank  two times, 10 days apart.

Clean the water frequently.  Oscars require spotless water.  Every week.  25% changed.

Filtration needs cleaned weekly as well.

Parasite tabs won't work as well as prazipro.  Use that.

Nitrites and ammonia should never be higher than 0, even though it might read "safe", it's not.

Happy fish-keeping.